Adsorbents with improved mass transfer properties and their use in the adsorptive separation of para-xylene

ABSTRACT

Adsorbents and methods for the adsorptive separation of para-xylene from a mixture containing at least one other C 8  aromatic hydrocarbon (e.g., a mixture of ortho-xylene, meta-xylene, para-xylene, and ethylbenzene) are described. Suitable adsorbents comprise zeolite X having an average crystallite size of less than 1.8 microns. The adsorbents provide improved mass transfer, which is especially advantageous for improving productivity in low temperature, low cycle time adsorptive separation operations in a simulated moving bed mode.

FIELD OF THE INVENTION

The present invention relates to adsorbents and methods for theadsorptive separation of para-xylene from a mixture containing at leastone other C₈ alkylaromatic hydrocarbon (e.g., a mixture of ortho-xylene,meta-xylene, para-xylene, and ethylbenzene). The adsorbents haveimproved mass transfer properties, which benefit the adsorptiveseparation process.

DESCRIPTION OF RELATED ART

C₈ alkylaromatic hydrocarbons are generally considered to be valuableproducts, with a high demand for para-xylene. In particular, theoxidation of para-xylene is used to commercially synthesize terephthalicacid, a raw material in the manufacture of polyester fabrics. Majorsources of para-xylene include mixed xylene streams that result from therefining of crude oil. Examples of such streams are those resulting fromcommercial xylene isomerization processes or from the separation of C₈alkylaromatic hydrocarbon fractions derived from a catalytic reformateby liquid-liquid extraction and fractional distillation. The para-xylenemay be separated from a para-xylene-containing feed stream, usuallycontaining a mixture of all three xylene isomers, by crystallizationand/or adsorptive separation. The latter technique has captured thegreat majority of the market share of newly constructed plants for theproduction of para-xylene.

Accordingly, numerous patents are directed to the adsorptive separationof para-xylene from feed streams containing a mixture of C₈alkylaromatics. Zeolites X and Y have been used to selectively adsorbpara-xylene. See, for example, U.S. Pat. No. 3,686,342, U.S. Pat. No.3,903,187, U.S. Pat. No. 4,313,015, U.S. Pat. No. 4,899,017, U.S. Pat.No. 5,171,922, U.S. Pat. No. 5,177,295, U.S. Pat. No. 5,495,061, andU.S. Pat. No. 5,948,950. U.S. Pat. No. 4,940,830 describes a rejectiveseparation of para-xylene from other xylene isomers and ethylbenzeneusing sodium zeolite Y or a sodium zeolite Y that is also ion-exchangedwith a Groups IB or Group VII element. A gas-phase process usingadsorptive separation to recover para-xylene from a mixture of xylenes,with an adsorbent comprising a crystalline molecular sieve having anaverage crystal size between 0.5 and 20 microns is described in WO2008/033200.

There remains a need in the art for improved adsorbents and processesfor the efficient separation of para-xylene from a relatively impuremixture of C₈ alkylaromatic hydrocarbons.

SUMMARY OF THE INVENTION

The invention relates to adsorbents that selectively adsorb para-xyleneover at least one other C₈ alkylaromatic compound present in a mixture.Due to the practical limitations of reaction equilibrium/selectivity, aswell as evaporative (distillation) separations, typical mixturesobtained from oil refining processes contain the other xylene isomers,ortho-xylene and meta-xylene (in addition to para-xylene), in varyingamounts and usually also contain ethylbenzene. Such mixtures willnormally constitute feed streams used in methods associated with theinvention.

Accordingly, embodiments of the invention are directed to processes forseparating para-xylene from a relatively impure mixture of one or moreC₈ alkylaromatic hydrocarbons other than the desired para-xylene. Themixture is contacted under adsorption conditions with an adsorbentcomprising zeolite X. Aspects of the invention are associated with thefinding that “small-crystallite-size zeolite X” (i.e., zeolite X havingan average crystallite size below 1.8 microns, and typically from about500 nanometers to about 1.5 microns) provides highly favorableperformance when incorporated into adsorbents used in the adsorptiveseparation of para-xylene. In particular, the mass transfer rate of (i)para-xylene into the zeolite pores during adsorption and (ii) desorbentinto the zeolite pores to displace adsorbed para-xylene duringdesorption, are significantly greater, relative to zeolite X synthesizedaccording to conventional methods (and typically having an averagecrystallite size of 1.8 microns or more).

This increase in mass transfer rate is especially advantageous in thecase of low temperature operation (less than about 175° C.), where masstransfer limitations associated with zeolite X-containing adsorbents,having conventional zeolite X crystallite sizes, are more commerciallysignificant. Low temperature operation is desirable for a number ofreasons, including increased para-xylene adsorptive selectivity andadsorbent capacity, as well as increased liquid feed density, all ofwhich directionally improve para-xylene productivity. Yet in a simulatedmoving bed mode of operation, which is often used in continuousindustrial processes for the adsorptive separation of para-xylene from afeed mixture of ortho-xylene, meta-xylene, para-xylene, andethylbenzene, these advantages associated with lower operatingtemperatures have been found to diminish as cycle time decreases, due tomass transfer limitations associated with para-xyleneadsorption/desorption. Adsorbents having improved mass transferproperties can therefore exploit the improvements in para-xylenecapacity and selectivity, as discussed above, associated with lowertemperature operation. Increased para-xylene productivity andconsequently improved process economics, are made possible.

Aspects of the invention relate to adsorbents comprisingsmall-crystallite-size zeolite X, as discussed above, which is boundwith a binder such as clay, alumina, silica, zirconia, or mixtures ofthese materials. In a representative embodiment, such a binder ispresent in an amount from about 10% to about 40% by weight, relative tothe total adsorbent weight.

The adsorbents comprising small-crystallite-size zeolite X may be usedin solid adsorbents employed in fixed bed, moving bed, or simulatedmoving bed adsorptive separation processes employing conventionaladsorption conditions. Adsorption may be performed in the liquid or gasphase, with liquid phase adsorption conditions normally being favored.When employed for the adsorptive separation of para-xylene in asimulated moving bed mode, the high mass transfer properties of theadsorbents described above allow for relatively increased para-xyleneproductivity, especially at in the case of low cycle time operation, incomparison to conventional adsorbents operating at the same overallpercentage of para-xylene recovery. That is, the adsorbent bedconcentration profiles are not adversely affected when cycle time is,for example, less than about 34 minutes (e.g., in the range from about24 minutes to about 34 minutes). The cycle time of a simulated movingbed adsorptive separation process refers to the time for any of theinlet or outlet streams to return to its original adsorbent bedposition. Therefore, in a typical simulated moving bed mode of operationwith 24 adsorbent beds (e.g., two vessels each having 12 beds), thecycle time refers, for example, to the time required for the inlet feedstream, initially introduced into the first bed at time zero, to againbe introduced to this bed. All other factors (e.g., para-xylene purityand recovery) being equal, shorter cycle times translate to higherproductivity.

Particular embodiments of the invention thus relate to a process forseparating para-xylene from a mixture comprising at least one other C₈alkylaromatic hydrocarbon, with the mixture normally containing thexylene isomers ortho- and meta-xylene as well as ethylbenzene. Theprocess comprises contacting the mixture with an adsorbent comprisingsmall-crystallite-size zeolite X having average zeolite crystallitesizes in the ranges as discussed above. Exemplary adsorptiontemperatures range from about 60° C. (140° F.) to about 250° C. (480°F.). However, by virtue of their improved mass transfer properties,operation at lower temperatures with these adsorbents does not imposethe appreciable mass transfer limitations associated with conventionaladsorbents. Therefore, as explained above, the benefits associated withimproved adsorptive para-xylene selectivity and adsorbent capacity atrelatively low temperatures can be more fully realized. Adsorptiontemperatures of less than about 175° C. (350° F.), for example fromabout 150° C. (300° F.) to about 175° C. (350° F.), are particularlyadvantageous when used with the adsorbents described above. Adsorptionpressures may range from slightly above atmospheric pressure, forexample about 1 barg (15 psig) to about 40 barg (580 psig).

The small-crystallite-size zeolite X used to formulate adsorbents forsuch adsorptive separations will generally have a molecular silica toalumina (SiO₂/Al₂O₃) molar ratio from about 2.0 to about 4.0,corresponding to an atomic Si/Al ratio from about 1.0 to about 2.0. Thezeolite normally has at least 95%, and typically substantially all (atleast 99%), of its ion-exchangeable sites exchanged with barium or acombination of barium and potassium. For example, a representativeadsorbent comprises small-crystallite-size zeolite X having from about60% to about 100% of its ion-exchangeable sites exchanged with bariumand from about 0% to about 40% of its ion-exchangeable sites exchangedwith potassium.

The contact between the mixture of C₈ alkylaromatic hydrocarbonsdescribed above (e.g., as a continuous or batch process feed stream)effects or brings about the adsorption of para-xylene into the zeolite Xpores, in preference to at least one other C₈ alkylaromatic hydrocarbonand normally in preference to all of such hydrocarbons present in themixture. Therefore, an adsorbed phase (i.e., within thesmall-crystallite-size zeolite X pores) will be selectively enriched inpara-xylene content, relative to that of the mixture of C₈ alkylaromaticcompounds (e.g., the feed stream). If the mixture contains ortho-xylene,meta-xylene, para-xylene, and ethylbenzene, then para-xylene will bepresent in the adsorbed phase, in an enriched amount relative to themixture, and ortho-xylene, meta-xylene, and ethylbenzene will be presentin the non-adsorbed phase, in enriched amounts relative to the mixture.

The non-adsorbed phase may then be removed from (or flushed) fromcontact with the adsorbent, for example in a raffinate stream. Theadsorbed phase, enriched in para-xylene, may be separately desorbed fromthe adsorbent, for example in an extract stream. A desorbent streamcomprising a desorbent, for example an aromatic ring-containing compoundsuch as toluene, benzene, indan, para-diethylbenzene,1,4-diisopropylbenzene, or a mixture thereof may be used for both theflushing and desorption. An exemplary adsorptive separation processutilizing adsorbents discussed above may be performed continuously in asimulated moving bed mode. According to this embodiment, a C₈alkylaromatic hydrocarbon feed stream as described above and thedesorbent stream are charged into a fixed bed of the adsorbent, whilethe extract and raffinate streams are removed from the bed. The chargingand removal of these streams may be performed continuously.

During a simulated moving bed mode or other type of adsorptiveseparation mode, it may be desired to monitor the water content of anoutlet stream, such as an extract or raffinate stream, in order todetermine the water content or level of hydration of the adsorbent. Ifnecessary, water may be added to an inlet stream, such as a feed streamand/or desorbent stream, either continuously or intermittently, tomaintain a desired level of adsorbent hydration (e.g., corresponding toa Loss in Ignition from about 5% to about 7%). Alternatively, water maybe added to obtain an absolute water content in the extract stream orraffinate stream, for example, from about 20 ppm by weight to about 120ppm by weight, corresponding to this adsorbent hydration level oranother desired adsorbent hydration level.

These and other aspects and features relating to the present inventionare apparent from the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the selectivities of feed mixture components (i.e., thepara-xylene/meta-xylene selectivity, “P/M”; the para-xylene/ortho-xyleneselectivity, “P/O”; and the para-xylene/ethylbenzene selectivity,“P/E”), as a function of temperature, obtained from a pulse test usingan adsorbent comprising zeolite X.

FIG. 2 shows the capacity of an adsorbent comprising zeolite X, as wellas the para-xylene/para-diethylbenzene selectivity, “PX/pDEB Sel,” as afunction of temperature, obtained from a breakthrough (or dynamic) test.

FIG. 3 shows the “DW” or “Delta W,” namely the half width of thepara-xylene peak (i.e., peak envelope width at half intensity), minusthe half-peak width of normal nonane (n-C₉) tracer peak, as a functionof temperature, obtained from a pulse test using an adsorbent comprisingzeolite X.

FIG. 4 shows the performance of a para-xylene adsorptive separationprocess operating in a simulated moving bed mode at temperatures of 150°C. (302° F.) and 177° C. (350° F.).

FIG. 5 shows the effect of cycle time on para-xylene recovery, in apara-xylene adsorptive separation process operating in a simulatedmoving bed mode, for an adsorbent comprising conventional zeolite Xcrystallites and another adsorbent comprising small-crystallite-sizezeolite X, as described above.

FIG. 6 shows the size distribution of both conventional zeolite Xcrystallites and zeolite X crystallites having a reduced size.

DETAILED DESCRIPTION

As discussed above, the invention relates to the separation ofpara-xylene from a mixture comprising at least one other C₈alkylaromatic hydrocarbon. The term separation refers to the recovery ofpara-xylene in a stream (e.g., a product stream) or fraction having anenriched para-xylene content (i.e., a content that is higher thaninitially present in the mixture). The separation is achieved throughcontacting the mixture with an adsorbent comprising zeolite X having anaverage crystallite size of less than 1.8 microns, and typically in therange from about 500 nanometers to about 1.5 microns.

Zeolite X is described in detail in U.S. Pat. No. 2,882,244.Small-crystallite-size zeolite X can be prepared from a seededsynthesis, where a seed or initiator material, used as a means ofnucleation or starting zeolite crystallite growth, is first prepared andthen blended into a gel composition at a gel composition: seed ratiocorresponding to a targeted crystallite size. The ratio of gelcomposition to seed governs the relative number or concentration ofnucleation sites, which in turn affects the crystallite size of thezeolite X that is synthesized. Higher amounts or concentrations of seeddirectionally reduce the crystallite size. For example, zeolite Xpreparations having average crystallite sizes of 2 microns and 0.5microns can be made using gel:seed ratios of about 5400:1 and 85:1, byweight, respectively. In view of the present disclosure, those havingskill in the art can readily vary the weight ratios to achieve otheraverage crystallite sizes. A typical gel composition comprises Na₂O,SiO₂, Al₂O₃, and water. For each mole of Al₂O₃, about 1-5 moles of Na₂Oand SiO₂, and about 100-500 moles of water, can be used in the gel.

The gel composition may be prepared by combining a gel makeup solutionwith an aluminate makeup solution containing, for example, about 12%alumina by weight. The gel makeup solution is prepared by mixing water,caustic solution, and sodium silicate, and cooling the mixture to about38° C. (100° F.). The aluminate makeup solution is prepared bydissolving alumina trihydrate in a caustic solution, with heating asnecessary for dissolution, followed by cooling and aging at about 38° C.(100° F.) prior to combining it with the gel makeup solution. The gelmakeup solution and aluminate solution are then combined under vigorousagitation for a short period (e.g., about 30 minutes), prior to addingthe required amount of seed.

The seed is prepared in a similar manner to the gel composition. Atypical seed composition therefore also comprises Na₂O, SiO₂, Al₂O₃, andwater. For each mole of Al₂O₃, about 10-20 moles of Na₂O and SiO₂, andabout 150-500 moles of water, can be used. The aluminate solution usedin preparing the seed may contain, for example, about 18% alumina byweight. After the gel composition and seed are combined, the mixture isheated while agitation is maintained, and then aged under agitatedconditions for a time from about 5 to about 50 hours and at atemperature from about 25° C. (75° F.) to about 150° C. (300° F.) toachieve the desired crystallite formation from the seed nuclei. Theresulting solid material may then by filtered, washed, and dried toobtain the prepared, small-crystallite-size zeolite X. Typically, thecrystallites of the small-particle-size zeolite X are bound with asuitable binder (e.g., an amorphous inorganic matrix such as clay,alumina, silica, zirconia, or mixtures thereof) into considerably largeradsorbent particles (e.g., in the range of about 16-60 Standard U.S.Mesh size) for use adsorptive separations. The process involvescombining the small-particle-size zeolite X with water and binder toprovide agglomerates, in the form of beads, extruded pellets, or otherforms that may be prepared by conventional methods such as thosedescribed in U.S. Pat. No. 4,818,508. The agglomerates are subjected tofiring at a temperature of about 500-700° C. to convert the greenagglomerates into stronger particles. Clays, including those comprisingboth silica and alumina, are exemplary materials for the non-zeolitic,amorphous binder of the adsorbent, which is present in intimate mixturewith the zeolite crystallites. Specific types of clays are attapulgite,minugel, and kaolin. The binder or matrix material may be an adjunct ofthe manufacturing process for the zeolite (e.g., from the intentionallyincomplete purification of the zeolite during its manufacture) or it maybe added to the relatively pure zeolite. In either case its usualfunction of the binder is to aid in forming the small-size zeolite Xcrystallites into hard adsorbent particles, such as extrudates,aggregates, tablets, macrospheres or granules having a desired particlesize range.

The zeolite will ordinarily be present in the adsorbent particles in anamount from about 75% to about 95% based on the volatile-free weight ofthe adsorbent composition. Volatile-free compositions are obtained bycalcining the adsorbent, for example, at 900° C.

Normally, the small-crystallite-size zeolite X that is incorporated intothe adsorbent particles is prepared in sodium form and the sodiumcations may be partially or wholly exchanged by different cations, suchas barium, potassium, strontium, and/or calcium, using known techniques.For example, small-crystallite-size zeolite X synthesized with at leastsome of its ion-exchangeable sites in sodium ion form may be immersed ina barium ion containing solution, or a barium and potassium ioncontaining solution, at conditions of time and temperature (e.g., about0.5 to about 10 hours at about 20 to about 125° C.) which can effect ionexchange or replacement of sodium ions with barium and/or potassiumions. Filtration of the zeolite, removal from the solution, andre-immersion in a fresh solution (e.g., having the same or differentratios or cations or other types of cations) can be repeated until adesired level of exchange, with the desired types and ratios of cations,is achieved. Ion-exchange can also be conducted in a column operationaccording to known techniques, for example by pumping preheated bariumchloride/potassium chloride solutions through a column of the adsorbentparticles to completely displace the sodium cations of thesmall-crystallite-size zeolite X. Normally, the small-crystallite-sizezeolite X used in adsorbents described herein have at least 95% orsubstantially all (e.g., at least 99%) of their ion-exchangeable sitesexchanged with barium or a combination of barium and potassium.Generally, no other metal ions occupy ion-exchangeable sites in anamount effective to alter the adsorptive properties of the zeolite. In arepresentative embodiment, small-crystallite-size zeolite X will havefrom about 60% to about 100% of its ion-exchangeable sites exchangedwith barium and from about 0% to about 40% of its ion-exchangeable sitesexchanged with potassium.

The number of ion-exchangeable sites decreases as the zeolite SiO₂/Al₂O₃molar ratio increases. Also, the total number of cations per unit celldecreases as monovalent cations (e.g., K⁺) are replaced by divalentcations (e.g., Ba⁺²). Within the zeolite X crystal structure, thereexist many ion-exchangeable site locations, some of these being inpositions outside of the supercages. Overall, the number and locationsof cations in the zeolite crystal structure will depend upon the sizesand numbers of the cations present, as well as the SiO₂/Al₂O₃ molarratio of the zeolite.

Those skilled in the art will recognize that the performance of anadsorbent (e.g., in terms of para-xylene purity and recovery into anextract stream) is influenced by a number of process variables,including operating conditions, feed stream composition, water content,and desorbent type and composition. The optimum adsorbent composition istherefore dependent upon a number of interrelated variables. In general,processes for the adsorptive separation of para-xylene from a mixturecontaining at least one other C₈ alkylaromatic hydrocarbon, as describedherein, can achieve high para-xylene purity (e.g., at least about 99wt-% or even at least about 99.5 wt-%) in the extract product streamwith a high overall recovery of para-xylene from the feed stream (e.g.,at least about 90%, from about 92% to about 99.5%, or from about 95% toabout 99%).

One consideration associated with overall adsorbent performance is itswater content, which may be determined from a Loss on Ignition (LOI)test that measures the weight difference obtained by drying a sample ofthe unused adsorbent at 900° C. under an inert gas purge such asnitrogen for a period of time (e.g., 2 hours) sufficient to achieve aconstant weight. The sample is first conditioned at 350° C. for 2 hours.The percentage of weight loss, based on the initial sample weight, isthe LOI at 900° C. An LOI from about 4% to about 7%, and preferably fromabout 4% to about 6.5%, by weight is generally desired for theadsorbents comprising small-crystallite-size zeolite X. An increased LOIvalue may be achieved by equilibrating a fixed amount of adsorbent,having a lower LOI than desired, and a fixed amount of water in a sealeddessicator until equilibrium is established. The LOI value of anadsorbent may be decreased through drying at 200-350° C. under an inertgas purge or vacuum for a period of time (e.g., 2 hours).

To monitor the adsorbent LOI during an adsorptive separation process, itmay be desired to determine the water content of a suitable outletstream of such a process, for example the raffinate stream and/orextract stream. Also, the adsorbent LOI may be adjusted or maintained,if desired, through continuous or intermittent addition or injection ofwater into a suitable inlet stream, for example the feed stream ordesorbent stream. According to one exemplary embodiment, the adsorbentLOI is maintained by monitoring the water content in the extract and/orraffinate streams. For example, a representative range of water ineither or both of these streams, corresponding to a desirable adsorbentLOI, is from about 20 ppm by weight to about 120 ppm by weight. Oftenthe desired water content in one or both of these outlet streams is fromabout 40 ppm to about 80 ppm by weight. Water may be added to either thefeed stream or the desorbent stream, as discussed above, in continuousor intermittent injections to maintain these measured water levels inthe extract stream, raffinate stream, or both.

It is recognized that LOI is an indirect or relative measurement of theadsorbent hydration level (or water content), as other volatilecomponents (e.g., organic materials) are also lost during the analysis.Therefore, the desired adsorbent water content is simply that whichcorresponds to an LOI from about 4% to about 7% by weight, measured asdescribed above. If necessary, the absolute amount of water in azeolitic adsorbent sample can be determined by known analytical methodssuch as Karl Fischer (ASTM D1364).

Although the adsorptive separation of para-xylene from other C₈alkylaromatic compounds may be conducted in either the liquid phase orthe vapor phase, predominantly liquid-phase operation is normally used.As discussed above, adsorption temperatures of less than about 175° C.(350° F.), for example from about 150° C. (300° F.) to about 175° C.(350° F.), are particularly advantageous when used with the adsorbentscomprising small-crystallite-size zeolite X, as mass transferlimitations, which prevent conventional adsorbents from exploiting theimproved para-xylene adsorptive selectivity and adsorbent capacity atthese temperatures, are overcome. Adsorption conditions can also includea pressure in the range from about atmospheric pressure to about 600psig, with pressures from about 1 barg (15 psig) to about 40 barg (580psig) being typical. Desorption conditions often include substantiallythe same temperature and pressure as used for adsorption.

Separation of para-xylene is carried out by contacting a mixture ofpara-xylene and at least one other C₈ alkylaromatic hydrocarbon with anadsorbent, and under adsorption conditions, as described above. Forexample, a feed stream comprising the mixture of C₈ alkylaromatichydrocarbons may be contacted with a bed of the adsorbent in order toselectively adsorb, in an adsorbed phase, the para-xylene, in preferenceto ortho-xylene, meta-xylene, and ethylbenzene. These other C₈alkylaromatic components of the feed stream may selectively pass throughthe adsorption zone as a non-adsorbed phase.

Feed streams comprising mixtures of C₈ alkylaromatic hydrocarbons can beseparated from various refinery process streams (e.g., reformate)including the products of separation units. Such separations areimprecise and the feed is therefore expected to contain limited amounts(e.g., less than 5 mol-%, and often less than 2 mol-%) of othercompounds, such as C₈ alkylaromatic hydrocarbons. In most instances, thefeed will be primarily C₈ alkylaromatic hydrocarbons and contain a totalof less than 10 mol-%, typically less than 5 mol-%, and in some casesless than 1 mol-%, of other types of compounds. In one type ofseparation process, after the adsorptive capacity of the adsorbent isreached, the feed stream inlet flow to the adsorbent is stopped, and theadsorption zone is then flushed to remove a non-adsorbed phase,initially surrounding the adsorbent, from contact with the adsorbent.The adsorbed phase, enriched in the desired para-xylene, may bethereafter desorbed from the adsorbent pores by treating the adsorbentwith a desorbent, normally comprising a cyclic hydrocarbon (e.g., anaromatic ring-containing hydrocarbon) such as toluene, benzene, indan,para-diethylbenzene, 1,4-diisopropylbenzene, or mixtures thereof. Thesame desorbent is commonly used for both (i) flushing the non-adsorbedphase into a raffinate stream comprising the desorbent and (ii)desorbing the adsorbed phase into an extract stream, also comprising thedesorbent. Because the extract stream contains the adsorbed phase, whichis enriched in para-xylene, the extract stream will also be enriched inpara-xylene, relative to the feed stream, when considered on adesorbent-free basis.

As used herein, a “feed stream” is a mixture containing the desiredextract component (para-xylene) and one or more raffinate components tobe separated by the adsorptive separation process. A “feed mixture”(i.e., comprising “feed mixture components”) therefore comprises themixture of extract and raffinate components, such as a mixture ofxylenes (ortho-xylene, meta-xylene, and para-xylene as discussed above)and ethylbenzene. The feed stream is an inlet stream to the adsorbent(e.g., in the form of one or more adsorbent beds) used in the process.An “extract component” is a compound or class of compounds that isselectively adsorbed by the adsorbent. A “raffinate component” is acompound or class of compounds that is less selectively adsorbed (orselectively rejected). A “desorbent” is generally any material capableof desorbing an extract component from the adsorbent, and a “desorbentstream” is an inlet stream to the adsorbent, which contains desorbent. A“raffinate stream” is an outlet stream from the adsorbent, in which araffinate component is removed. The composition of the raffinate streamcan vary from essentially 100% desorbent to essentially 100% raffinatecomponents, with minor amounts one or more extract components. An“extract stream” is an outlet stream from the adsorbent, in which anextract component is removed. The composition of the extract stream canvary from essentially 100% desorbent to essentially 100% extractcomponents, with minor amounts of one or more raffinate components.

Typically, at least some portion of the extract stream and the raffinatestream are purified (e.g., by distillation) to remove desorbent andthereby produce an extract product stream and a raffinate productstream. The “extract product stream” and “raffinate product stream”refer to products produced by the adsorptive separation processcontaining, respectively, an extract component and a raffinate componentin higher concentration than present in the extract stream and theraffinate stream, respectively, and also in higher concentration thanpresent in the feed stream.

The capacity of the adsorbent for adsorbing a specific volume of anextract component is an important characteristic, as increased capacitymakes it possible to reduce the amount of adsorbent (and consequentlythe cost) needed to separate the extract component for a particularcharge rate of feed mixture. Good initial capacity for the extractcomponent, as well as total adsorbent capacity, should be maintainedduring actual use in an adsorptive separation process over someeconomically desirable life.

The rate of exchange of an extract component (para-xylene) with thedesorbent can generally be characterized by the width of the peakenvelopes at half intensity obtained from plotting the composition ofvarious species in the adsorption zone effluent obtained during a pulsetest versus time. The narrower the peak width, the faster the desorptionrate. The desorption rate can also be characterized by the distancebetween the center of the tracer peak envelope and the disappearance ofan extract component which has just been desorbed. This distance is timedependent and thus a measure of the volume of desorbent used during thistime interval. The tracer is normally a relatively non-adsorbed compoundwhich moves through an adsorbent column faster than the materials to beseparated.

Selectivity (β), for an extract component with respect to a raffinatecomponent may be characterized by the ratio of the distance between thecenter of the extract component peak envelope and the tracer peakenvelope (or other reference point) to the corresponding distancebetween the center of the raffinate component peak envelope and thetracer peak envelope (or other reference point). The selectivitycorresponds to the ratio of the two components in the adsorbed phasedivided by the ratio of the same two components in the non-adsorbedphase at equilibrium conditions. Selectivity may therefore be calculatedfrom:

Selectivity=(wt-% C _(A)/wt-% D _(A))/(wt-% C _(U)/wt-% D _(U))

where C and D are two components of the feed mixture represented inweight percent and the subscripts A and U represent the adsorbed andnon-adsorbed phases, respectively. The equilibrium conditions aredetermined when the feed passing over a bed of adsorbent does not changecomposition, in other words, when there is no net transfer of materialoccurring between the non-adsorbed and adsorbed phases. In the equationabove, a selectivity larger than 1.0 indicates preferential adsorptionof component C within the adsorbent. Conversely, a selectivity less than1.0 would indicate that component D is preferentially adsorbed leavingan non-adsorbed phase richer in component C and an adsorbed phase richerin component D.

For a selectivity of two components approaching 1.0, there is nopreferential adsorption of one component by the adsorbent with respectto the other (i.e., they are both adsorbed to about the same degree withrespect to each other). As selectivity deviates from 1.0, there is anincreasingly preferential adsorption by the adsorbent for one componentwith respect to the other. Selectivity can be expressed not only for onefeed stream compound relative to another (e.g., para-xylene tometa-xylene selectivity) but can also be expressed between any feedstream compound and the desorbent (e.g., para-xylene topara-diethylbenzene selectivity).

While separation of an extract component from a raffinate component istheoretically possible when the adsorbent selectivity for the extractcomponent with respect to the raffinate component is only slightlygreater than 1, it is preferred that this selectivity is at least 2 forprocess economic considerations. Analogous to relative volatility infractional distillation, the higher the selectivity, the easier theadsorptive separation is to perform. Higher selectivities directionallypermit a smaller amount of adsorbent to be used, just as higher relativevolatilities require fewer theoretical stages of distillation (and asmaller column) to carry out a given distillation separation for a givenfeed.

The desorbent for an adsorptive separation process must be judiciouslyselected to satisfy several criteria. The desorbent should ideallydisplace an extract component from the adsorbent at a reasonable massflow rate, without itself being so strongly adsorbed as to prevent anextract component from displacing the desorbent in a followingadsorption cycle. In terms of the selectivity, it is preferred that theadsorbent be more selective for the extract component with respect to araffinate component than it is for the desorbent with respect to theraffinate component. Additionally, the desorbent must be compatible withboth the adsorbent as well as the feed mixture. In particular, thedesorbent should not adversely effect the desired selectivity of theadsorbent for an extract component with respect to a raffinatecomponent. Additionally, the desorbent should be essentially chemicallyinert with respect to extract and raffinate components, as both theextract stream and the raffinate stream are typically removed from theadsorbent in admixture with desorbent. Any chemical reaction involvingdesorbent and an extract component or a raffinate component wouldcomplicate or possibly prevent product recovery.

A performance parameter to be considered for the desorbent is thereforeits rate of exchange for the extract component of the feed or, in otherwords, the relative rate of desorption of the extract component. Thisparameter relates directly to the amount of desorbent that must be usedin an adsorptive separation process to desorb the extract component fromthe adsorbent. Faster rates of exchange reduce the amount of desorbentneeded and therefore reduce operating costs associated with largerdesorbent-containing process streams, including the separation andrecycle of desorbent from these streams. A desorbent selectivity of 1 orslightly lower with respect to an extract component helps ensure thatall the extract component is desorbed with a reasonable flow rate ofdesorbent, and also that extract components can displace desorbent in asubsequent adsorption step. As a final consideration, the desorbentshould generally be readily available at a favorable cost.

Moreover, since both the raffinate stream and the extract streamnormally contain desorbent, the desorbent should also be easilyseparable from the mixture of extract and raffinate componentsintroduced in the feed stream. Without a method of separating desorbentin the extract stream and the raffinate stream, the concentration of anextract component in the extract product and the concentration of araffinate component in the raffinate product would not be commerciallyfavorable, nor would the desorbent be available for reuse in theprocess. At least a portion of the desorbent is therefore normallyrecovered from the extract stream and the raffinate stream of anadsorptive separation process by distillation or evaporation, althoughother separation methods such as reverse osmosis could also be usedalone or in combination with distillation or evaporation. In thisregard, the desorbent is generally a “light” or “heavy” desorbent thatis easily separable, as a distillation overhead or bottoms product,respectively, from the C₈ alkylaromatic hydrocarbons in the extractstream and raffinate stream of a para-xylene adsorptive separationprocess.

A “pulse test” may be employed to test adsorbents with a particular feedmixture and desorbent to measure such adsorbent properties as adsorptivecapacity, selectivity, resolution, and exchange rate. A representativepulse test apparatus includes a tubular adsorbent chamber ofapproximately 70 cubic centimeters (cc) in volume and having inlet andoutlet portions at opposite ends of the chamber. The chamber is equippedto allow operation at constant, predetermined temperature and pressure.Quantitative and qualitative analytical equipment such asrefractometers, polarimeters and chromatographs can be attached to anoutlet line of the chamber and used to detect quantitatively and/ordetermine qualitatively one or more components in the effluent streamleaving the adsorbent chamber. During a pulse test, the adsorbent isfirst filled to equilibrium with a particular desorbent by passing thedesorbent through the adsorbent chamber. A small volume or pulse of thefeed mixture, which may be diluted with desorbent, is injected byswitching the desorbent flow to the feed sample loop at time zero.Desorbent flow is resumed, and the feed mixture components are eluted asin a liquid-solid chromatographic operation. The effluent can beanalyzed on-stream or, alternatively, effluent samples can be collectedperiodically and analyzed separately (off-line) and traces of theenvelopes of corresponding component peaks plotted in terms of componentconcentration versus quantity of effluent.

Information derived from the pulse test can be used to determineadsorbent void volume, retention volume for an extract component or araffinate component, selectivity for one component with respect to theother, stage time, the resolution between the components, and the rateof desorption of an extract component by the desorbent. The retentionvolume of an extract component or a raffinate component may bedetermined from the distance between the center of the peak envelope ofan extract component or a raffinate component and the peak envelope of atracer component or some other known reference point. It is expressed interms of the volume in cubic centimeters of desorbent pumped during thetime interval corresponding to the distance between the peak envelopes.

Retention volumes for good candidate systems fall within a range set byextrapolation to commercial designs. A very small retention volumeindicates there is little separation between the two components (i.e.,one component is not adsorbed strongly enough). Large extract componentretention volumes indicate it is difficult for the desorbent to removethe retained extract component.

Conventional apparatuses employed in static bed fluid-solid contactingmay be used in adsorptive separation processes incorporating anadsorbent comprising small-crystallite-size zeolite X, as describedabove. The adsorbent may be employed in the form of a single fixed bedwhich is alternately contacted with the feed stream and desorbentstream. The adsorbent may therefore be used in a single static bed thatis alternately subjected to adsorption and desorption steps in anon-continuous (e.g., batch) process. A swing bed mode of operation isalso possible, in which multiple beds are periodically used for a givenoperation or step.

Alternatively, a set of two or more static beds may be employed withappropriate piping/valves to allow continual passage of the feed streamthrough any one of a number of adsorbent beds while the desorbent streamis passed through one or more of the other beds in the set. The flow ofthe feed stream and desorbent may be either upward or downward throughthe adsorbent.

A countercurrent moving bed mode of operation provides another potentialmode of operation, in which a stationary concentration profile of thefeed mixture components can be achieved, allowing for continuousoperation with fixed points of feed stream and desorbent streamintroduction, as well as extract stream and raffinate stream withdrawal.Countercurrent moving bed or simulated moving bed countercurrent flowsystems have a much greater separation efficiency than fixed adsorbentsystems and are therefore very often used for commercial-scaleadsorptive separations. In a simulated moving bed process, theadsorption and desorption are carried out continuously in a simulatedmoving bed mode, allowing both continuous production (withdrawal) of anextract stream and a raffinate stream (both outlet streams), as well asthe continual use (input) of a feed stream and a desorbent stream (bothinlet streams).

The operating principles and step sequence of a simulated moving bedflow system are described in U.S. Pat. No. 2,985,589, U.S. Pat. No.3,310,486, and U.S. Pat. No. 4,385,993, incorporated by reference hereinfor their teachings with respect to simulated moving bed flow systems.In such systems, it is the progressive movement of multiple accesspoints along an adsorbent chamber that simulates the movement ofadsorbent (opposite the liquid access point movement) contained in oneor more chambers. Typically only four of the many (16 to 24 or more)access lines to the chamber(s) are active at any one time, for carryingthe feed stream, the desorbent stream, the raffinate stream, and theextract stream. Coincident with this simulated movement (e.g., upwardmovement) of the solid adsorbent is the movement (e.g., downwardmovement) of fluid occupying the void volume of the packed bed ofadsorbent. The circulation of this fluid (e.g., liquid) flow may bemaintained using pump. As an active liquid access point moves through acycle, that is, through each adsorbent bed contained in one or morechambers, the chamber circulation pump generally provides different flowrates. A programmed flow controller may be provided to set and regulatethese flow rates.

The active access points effectively divide the adsorbent chamber intoseparate zones, each of which has a different function. Three separateoperational zones are generally present for the process to take place,although, in some cases, an optional fourth operation zone is used. Thezone numbers used in the following description of a simulated moving bedprocess correspond to those illustrated in U.S. Pat. No. 3,392,113 andU.S. Pat. No. 4,475,954, also incorporated by reference herein withrespect to their teachings regarding simulated moving bed operation.

The adsorption zone (zone 1) is defined as the adsorbent located betweenthe inlet feed stream and the outlet raffinate stream. In this zone, thefeed mixture contacts the adsorbent, an extract component is adsorbed,and a raffinate stream is withdrawn. The general flow through zone 1 isfrom the feed stream which passes into the zone to the raffinate streamwhich passes out of the zone, and the flow in this zone is normallyconsidered to be in a downstream direction when proceeding from theinlet feed stream to the outlet raffinate stream.

Immediately upstream, with respect to fluid flow in zone 1, is thepurification zone (zone 2).

The purification zone is defined as the adsorbent between the outletextract stream and the inlet feed stream. The basic operations in thiszone are the displacement from the non-selective void volume of theadsorbent of any raffinate component carried into zone 2 and thedesorption of any raffinate component adsorbed within the selective porevolume of the adsorbent or adsorbed on the surfaces of the adsorbentparticles. Purification is achieved by passing a portion of the extractstream leaving zone 3 into zone 2 at its upstream boundary, the extractoutlet stream, to effect the displacement of raffinate components. Theflow in zone 2 is in a downstream direction from the extract outletstream to the feed inlet stream. This material then joins the feedstream and flows through zone 1.

Immediately upstream of zone 2 with respect to the fluid flowing in zone2 is the desorption zone (zone 3). The desorption zone is defined as theadsorbent between the inlet desorbent stream and the outlet extractstream. The function of the desorbent zone is to allow a desorbent whichpasses into this zone to displace the extract component which wasadsorbed in the adsorbent during a previous contact with the feedmixture in zone 1, in a prior cycle of operation. The flow of fluid inzone 3 is essentially in the same direction as that in zones 1 and 2.

In some instances, an optional buffer zone (zone 4) may be utilized.This zone, defined as the adsorbent between the outlet raffinate streamand the inlet desorbent stream, if used, is located immediately upstreamwith respect to the fluid flow into zone 3. Zone 4 can be utilized toconserve the amount of desorbent needed for desorption, since a portionof the raffinate stream which is removed from zone 1 can be passed intozone 4 to displace desorbent from that zone out into the desorptionzone. Zone 4 will contain enough adsorbent so that raffinate componentsin the raffinate stream passing from zone 1 into zone 4 can be preventedfrom passing into zone 3 and thereby contaminating the extract streamwithdrawn from zone 3. If a fourth operational zone is not utilized, theraffinate stream passed from zone 1 to zone 4 must be carefullymonitored in order that the flow directly from zone 1 to zone 3 can bestopped when there is an appreciable quantity of raffinate componentspresent in the raffinate stream passing from zone 1 into zone 3 so thatthe extract outlet stream is not contaminated.

A cyclic advancement of the input (feed and desorbent) streams andoutput (extract and raffinate) streams through fixed beds of adsorbent,to provide a continuous process performed in a simulated moving bedmode, can be accomplished by utilizing a manifold system in which thevalves in the manifold are operated in a manner which effects theshifting of the input and output streams thereby providing a flow offluid with respect to solid adsorbent in a simulated countercurrentmanner. Another type of operation which can simulate countercurrent flowof solid adsorbent involves the use of a rotating valve in which theinput and output streams are each directed by the valve to one of themany lines connected to the adsorbent chamber and by which the locationat which the input feed stream, output extract stream, input desorbentstream, and output raffinate stream enter or leave the chamber areadvanced in the same direction along the adsorbent bed. Both themanifold arrangement and rotary disc valve are known in the art. Amultiple valve apparatus is described in detail in U.S. Pat. No.4,434,051. Rotary disc valves which can be utilized in this operationare described in U.S. Pat. No. 3,040,777, U.S. Pat. No. 4,632,149, U.S.Pat. No. 4,614,204, and U.S. Pat. No. 3,422,848. These patents disclosea rotary type valve in which the suitable advancement of the variousinput and output streams from fixed sources can be achieved withoutdifficulty.

In many instances, one operational zone of a simulated moving bedprocess will contain a much larger quantity of adsorbent than anotheroperational zone. For instance, in some operations the buffer zone cancontain a minor amount of adsorbent compared to the adsorbent present inthe adsorption and purification zones. As another example, in instancesin which a desorbent is used that easily desorbs the extract componentfrom the adsorbent, a relatively small amount of adsorbent will beneeded in the desorption zone compared to the amount of adsorbent neededin the buffer zone, adsorption zone, purification zone or all of thesezones. Also, it is not required that the adsorbent be located in asingle chamber (i.e., column or vessel), and often two adsorbentchambers (e.g., each provided with 12 access lines) are used. Additionalchambers are also contemplated.

Normally at least a portion of the output extract stream will pass to aseparation process such as a fractionation column, in order to recover aportion of the desorbent (e.g., for recycle to the adsorptive separationprocess as a desorbent recycle stream) and produce a purified extractproduct stream (e.g., containing a reduced amount of desorbent).Preferably at least a portion of the output raffinate stream will alsopass to a separation process, in order to recover another portion of thedesorbent (e.g., also for recycle to the adsorptive separation process)and a raffinate product stream (e.g., also containing a reducedconcentration of desorbent). In large-scale petrochemical units,essentially all of the desorbent is recovered for reuse. The design offractional distillation facilities for this recovery will be dependenton the materials being separated, the desorbent composition, etc.

Another type of a simulated moving bed flow system suitable for use inadsorptive separation processes described above is a co-current highefficiency simulated moving bed process described in U.S. Pat. No.4,402,832 and U.S. Pat. No. 4,478,721, incorporated by reference hereinwith respect to their teachings of this alternative mode of operation.This process has advantages in terms of energy efficiency and reducedcapital costs, in cases where products of slightly lower purity areacceptable to the producer.

The scale of adsorptive separation units for the purification ofpara-xylene can vary from those of pilot plant scale (see, for example,U.S. Pat. No. 3,706,812) to commercial scale and can range in produceflow rates from as little as a few milliliters an hour to many hundredsof cubic meters per hour.

Overall, aspects of the invention are directed to the exploitation ofthe unique properties of small-crystallite-size zeolite X for use inadsorptive separations, and particularly in commercial simulated movingbed operations. In view of the present disclosure, it will be seen thatseveral advantages may be achieved and other advantageous results may beobtained. Those having skill in the art, with the knowledge gained fromthe present disclosure, will recognize that various changes could bemade in the above adsorbent compositions, and processes using thesecompositions, without departing from the scope of the presentdisclosure. The chemical processes, mechanisms, modes of interaction,etc. used to explain theoretical or observed phenomena or results, shallbe interpreted as illustrative only and not limiting in any way thescope of the appended claims.

The following examples are set forth as representative of the presentinvention. These examples are not to be construed as limiting the scopeof the invention as these and other equivalent embodiments will beapparent in view of the present disclosure and appended claims.

EXAMPLE 1 Selectivity Testing

A conventional adsorbent comprising zeolite X exchanged with barium andpotassium, and having an LOI of 6 wt-%, was evaluated in the adsorptiveseparation of para-xylene. A standard pulse test as described above wasperformed by first loading the adsorbent in a 70 cc column under thedesorbent para-diethylbenzene. A feed pulse containing equal quantitiesof ethylbenzene and each of the three xylene isomers, together with anormal nonane (n-C₉) tracer, was injected. Pulse tests were performed atvarious column temperatures in the range from 121° C. to 177° C. (250°F. to 350° F.) to examine the effect of temperature on selectivity. Thepara-xylene selectivities were determined from the component peaksobtained from each of these pulse tests, and the results are shown inFIG. 1.

The results show that the para-xylene/meta-xylene selectivity, “P/M,”and the para-xylene/ortho-xylene selectivity, “P/O,” increase at loweradsorption temperatures, while the para-xylene/ethylbenzene selectivity,“P/E,” varies little with respect to temperature. This demonstrates thatlower adsorptive separation temperatures are favored from athermodynamic equilibrium (although not necessarily from a kinetic)standpoint.

EXAMPLE 2 Capacity Testing

A chromatographic separation (dynamic capacity or breakthrough test) wasperformed using a conventional adsorbent as described in Example 1. Acolumn containing 70 cc of the adsorbent, initially loaded underpara-diethylbenzene, was charged with a sample feed mixture containingortho-xylene, meta-xylene, para-xylene, and ethylbenzene. Breakthroughtests were performed at various column temperature in the range from121° C. to 177° C. (250° F. to 350° F.) to examine the effect oftemperature on adsorbent capacity and para-xylene/para-diethylbenzeneselectivity, “PX/pDEB Sel,” and the results are shown in FIG. 2. Theresults further confirm that lower adsorptive separation temperaturesare favorable, in terms of total adsorbent capacity, without adverselyaffecting the para-xylene/desorbent selectivity.

EXAMPLE 3 Mass Transfer Rate

Using the data obtained from pulse tests as described in Example 1, the“DW” or “Delta W,” namely the half width of the para-xylene peak (i.e.,peak envelope width at half intensity), minus the half width of normalnonane tracer peak (to account for dispersion), was determined as afunction of temperature. The results are shown in FIG. 3 and indicatethat mass transfer limitations become significant in conventionalpara-xylene adsorptive separation processes at temperatures below about177° C. (350° F.).

EXAMPLE 4

Adsorptive Separation of Para-Xylene in Simulated Moving Bed Mode,Temperature Effects

The performance of a conventional adsorbent, as described in Example 1,in the adsorptive separation of para-xylene in a pilot plant operatingin a simulated moving bed mode is illustrated in FIG. 4. At allconditions analyzed, the total para-xylene recovered in the extractstream was about 95 wt-%. Measured performance parameters at 150° C.(302° F.) are shown (solid square symbols), compared to those at 177° C.(350° F.) (solid triangular symbols), as a function of the process cycletime. In all cases, the value of zeta (Y-axis, left side) at the lowertemperature was below the corresponding value at the higher temperature,indicating that higher para-xylene productivity was achieved at loweroperating temperature, due to the improved para-xylene adsorptionselectivity and adsorbent capacity, as confirmed in the pulse andbreakthrough tests, described in Examples 1 and 2.

The zeta value is a measure of selective pore volume required forprocessing a unit volumetric rate of para-xylene containing feed. Lowerzeta values therefore directionally indicate higher throughput orproductivity. In particular, the zeta value is calculated as the productof A/F_(a) and θ, where A is the rate of simulated circulation of theselective adsorbent pore volume, F_(a) is the volumetric rate of thefeed stream containing the mixture of C₈ alkylaromatic hydrocarbons, andθ is the cycle time. The quantity A/F_(a) is shown in FIG. 4 for eachcombination of cycle time and temperature tested. The near convergence,at low cycle times, of the curves generated at high and low temperatureoperation indicates that the low temperature performance advantagesbecome less pronounced as cycle time is reduced (and as productivity orzeta is maximized).

This barrier to obtaining even higher productivity, observed inpractice, is due to mass transfer limitations. The bottom curve in FIG.4 (open diamonds) illustrates the impact of mass transfer limitations onproductivity, in terms of the possible feed stream volumetric rateincrease (Y-axis, right side) that is obtained by reducing operatingtemperature from 177° C. (350° F.) to 150° C. (302° F.) at each cycletime. For a cycle time of 34 minutes, for example, the benefit of loweroperating temperature translates into an approximately 14% increase inthe volumetric rate at which the C₈ alkylaromatic feed stream that canbe processed. The productivity advantage, however, decreases to onlyabout 7% for a cycle time of 24 minutes, as a result of mass transferlimitations. The dashed lines in FIG. 4, in contrast, illustrate thepara-xylene adsorptive separation performance parameters and feedproductivity advantages that are theoretically possible in the absenceof mass transfer limitations.

The laboratory and pilot plant scale studies described in Examples 1-4therefore illustrate that, while lower operation temperature results insignificant advantages in adsorbent capacity, adsorption selectivity,and increased liquid density per unit volume of adsorbent, it alsoreduces the mass transfer rate. As a result, it is difficult to takefull advantage of the thermodynamically favorable operating regime, attemperatures ranging from about 150° C. (300° F.) to about 175° C. (350°F.), in the adsorptive separation of para-xylene from a relativelyimpure mixture of C₈ alkylaromatic hydrocarbons. As is shown in FIG. 4,with decreasing cycle time, the productivity advantage of 150° C. (300°F.) operation decreases in the case of conventional adsorbents.

EXAMPLE 5 Adsorptive Separation of Para-Xylene in Simulated Moving BedMode, Bed Concentration Profiles

The feed mixture component and desorbent concentration profiles for eachof 24 adsorbent beds, obtained during the adsorptive separation ofpara-xylene in a pilot plant operating in a simulated moving bed modeusing a conventional adsorbent (as described in Example 1) wereevaluated. A liquid composition “survey” (i.e., analysis of the liquidcomposition in each bed) was performed during steady state operation at150° C. (302° F.) and about 95% recovery of para-xylene in the extractstream. The concentration profiles were obtained using a cycle time of34 minutes and 24 minutes, respectively. The survey results confirmedthat significant mass transfer limitations prevent full realization ofthe more favorable para-xylene adsorption selectivity and adsorbentcapacity at this temperature. The adverse effects of limited masstransfer were manifested in a more dispersed para-xylene front in theadsorption zone (zone I, as discussed above). The theoreticalproductivity increase associated with low temperature operation cannotbe realized, using a conventional adsorbent, as cycle time is reduced tomore commercially desirable values (e.g., 24-34 minutes).

High Resolution Scanning Electron Microscopy (HR SEM) micrographs ofadsorbents comprising zeolite X (with differing amounts of binder) alsoillustrated that the zeolite X crystallites present the main diffusionpath that limits the mass transfer rate.

EXAMPLE 6 Use of Adsorbent Comprising Small-Crystallite-Size Zeolite Xto Improve Adsorptive Separation of Para-Xylene in Simulated Moving BedMode

FIG. 5 compares the performance of a conventional adsorbent comprisingzeolite X, having an average crystallite of size 1.8-2.0 microns, to anadsorbent comprising zeolite X, having an average crystallite size of1.4 microns. The zeolite X crystallite size distributions, for arepresentative conventional zeolite X sample and another sample havingthe reduced crystallite size, such as those used in the testedadsorbents, are illustrated in FIG. 6, with the average crystallite sizefor the conventional zeolite X being 1.8-2.0 microns. While maintainingall other conditions constant in a para-xylene adsorptive separationprocess operating an a simulated moving bed mode, the adsorbentcomprising small-crystallite-size zeolite X allowed for a higher overallpara-xylene recovery. The advantages associated with the smallcrystallite size zeolite X, related to the improved adsorbent masstransfer characteristics, became more pronounced as cycle time wasreduced, for the reasons discussed above, and as demonstrated in FIG. 5.

Thus, the reduction in zeolite X crystallite size in adsorbents provideda shortened mass transfer path, and consequently an improved masstransfer rate. This has important commercial implications, aspara-xylene producers strive to improve productivity by reducing cycletime in adsorptive separation processes operating in simulated movingbed mode. For example, many commercial operations are conducted using30- or even 27-minute cycle times.

In these cases, the use of adsorbents comprising small-crystallite-sizezeolite X is especially advantageous in overcoming mass transfer ratelimitations observed for operating temperatures of less than about 175°C. (350° F.). Small-crystallite-size zeolite X has been synthesized withaverage crystallite sizes in the range from about 0.5 microns (500nanometers) to about 1.4 microns, which allows for optimization of thezeolite crystallite size for a given application, and consequently theadsorbent formulation and overall process.

1. A process for separating para-xylene from a mixture comprising atleast one other C₈ alkylaromatic hydrocarbon, the process comprisingcontacting, under adsorption conditions, the mixture with an adsorbentcomprising zeolite X having an average crystallite size of less than 1.8microns.
 2. The process of claim 1, wherein the average crystallite sizeis from about 500 nanometers to about 1.5 microns.
 3. The process ofclaim 1, wherein the adsorption conditions include an adsorptiontemperature of less than about 175° C. (350° F.).
 4. The process ofclaim 3, wherein the adsorption conditions include and adsorptiontemperature from about 150° C. (300° F.) to about 175° C. (350° F.). 5.The process of claim 1, wherein the adsorption conditions include anadsorption pressure from about 1 barg (15 psig) to about 40 barg (580psig).
 6. The process of claim 1, wherein the adsorbent comprises abinder comprising a material selected from the group consisting of clay,alumina, silica, zirconia, and mixtures thereof.
 7. The process of claim6, wherein the binder is present in an amount from about 10% to about40% by weight, relative to the adsorbent.
 8. The process of claim 1,wherein the adsorbent has a water content corresponding to a Loss onIgnition from about 4% to about 7% by weight.
 9. The process of claim 1,wherein the zeolite X has at least 95% of its ion-exchangeable sitesexchanged with barium or a combination of barium and potassium.
 10. Theprocess of claim 1, wherein the mixture comprises ortho-xylene,meta-xylene, para-xylene, and ethylbenzene.
 11. The process of claim 10,wherein contacting the mixture with the adsorbent effects adsorption ofpara-xylene, present in an adsorbed phase, in preference toortho-xylene, meta-xylene, and ethylbenzene, present in a non-adsorbedphase.
 12. The process of claim 11, further comprising: flushing thenon-adsorbed phase from contact with the adsorbent, and desorbingpara-xylene in the adsorbed phase from the adsorbent.
 13. The process ofclaim 12, wherein para-xylene in the adsorbed phase is desorbed into anextract stream comprising a desorbent and the non-adsorbed phase isflushed into a raffinate stream comprising the desorbent.
 14. Theprocess of claim 13, wherein the desorbent comprises an aromaticring-containing compound selected from the group consisting of toluene,benzene, indan, para-diethylbenzene, 1,4-diisopropylbenzene, andmixtures thereof.
 15. The process of claim 14, wherein the process isperformed continuously in a simulated moving bed mode, wherein a feedstream and a desorbent stream are charged into a bed of the adsorbent,the feed stream comprising the mixture comprising ortho-xylene,meta-xylene, para-xylene, and ethylbenzene, and the desorbent streamcomprising the desorbent; and wherein the extract stream and theraffinate stream are removed from the bed of the adsorbent.
 16. Theprocess of claim 15, wherein the process has a cycle time of less thanabout 34 minutes.
 17. The process of claim 16, wherein the cycle time isfrom about 24 minutes to about 34 minutes.
 18. A process for separatingpara-xylene from a mixture comprising ortho-xylene, meta-xylene,para-xylene, and ethylbenzene, wherein the process is performed in asimulated moving bed mode and comprises: charging a feed streamcomprising the mixture and a desorbent stream into one or more vesselscomprising an adsorbent comprising zeolite X having an averagecrystallite size of less than 1.8 microns, desorbing para-xylene in theadsorbed phase into an extract stream comprising the desorbent, flushingthe non-adsorbed phase into a raffinate stream comprising the desorbent,and removing the extract stream and the raffinate stream from the bed ofthe adsorbent, wherein the process has a cycle time from about 24minutes to about 34 minutes and is performed at a temperature of lessthan about 175° C. (350° F.).
 19. The process of claim 18, wherein thezeolite X has a SiO₂/Al₂O₃ framework molar ratio from about 2.0 to about3.0.
 20. The process of claim 18, wherein water is added to either thefeed stream or the desorbent stream to obtain a water content in theextract or raffinate stream from about 20 ppm by weight to about 120 ppmby weight.